The present invention relates to fine particles obtained by substituting a part of Fe(III) in plate-like particles of barium ferrite with Co(II) and Ti(IV), for use in magnetic recording, and a process for producing the same.
More in detail, the present invention relates to plate-like barium ferrite particles for magnetic recording, having BET specific surface area of 20 to 70 m.sup.2 /g and average particle diameter of 0.05 to 0.3 .mu.m, and showing magnetization of larger than 30 emu/g in a magnetic field of 10 KOe and coercive force of 300 to 1,000 Oe, represented by the formula BaCo.sub.x Ti.sub.x Fe.sub.12-2x O.sub.19 wherein x is 0.1 to 0.5 and to a process for producing the plate-like barium ferrite particles represented by the formula: EQU BaCo.sub.x Ti.sub.x Fe.sub.12-2x O.sub.19
wherein x is 0.1 to 0.5 which comprises autoclaving an aqueous highly alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions at a temperature of 250.degree. to 320.degree. C., the atomic ratio of total amount of Co(II) and Ti(IV) to the amount of Fe(III) being 0.017:1 to 0.09:1, the amount of Co(II) being the same as that of Ti(IV) and the atomic ratio of Ba ions to the total sum of Fe(III), Co(II) and Ti(IV) being 1:7 to 1:9.
In recent years, accompanying with the propagation of VTR, audio component, word processer and computer, non-acicular ferromagnetic particles of a favorable dispersibility provided with a suitable coercive force(Hc) have been required as the magnetic material for recording, particularly as the magnetic material for the perpendicular magnetic recording.
In general, as the ferromagnetic non-acicular particles, barium ferrite particles have been well known. However, the coercive force of barium ferrite particles obtained by the dry method is ordinarily larger than 3,000 Oe, and because of such a high coercive force, barium ferrite particles are not favorable as a magnetic material for magnetic recording.
Further, the conventional barium ferrite particles obtained by sintering are composed of polycrystals of an average diameter of a few micrometers, and even after pulverizing thereof, the average diameter of the thus pulverized particles is around one micrometer. Accordingly, such barium ferrite particles are poor in dispersibility in paints and is not favorable as a magnetic material for magnetic recording.
Namely, as a magnetic material for magnetic recording, those which are as fine as possible, particularly those of an average particle diameter of 0.05 to 0.3 .mu.m have been demanded. For instance, Japanese Patent Application Laying Open No. 53-20596 (1978) discloses that there is a difficulty in uniformly dispersing the particles of diameter over 0.5 .mu.m. Japanese Patent Application Laying Open No. 56-125219 (1981) discloses as follows. "In the range of recording wavelength of shorter than 1 .mu.m, perpendicular magnetic recording is more useful as compared to longitudinal magnetic recording, and in order to effect the sufficient recording and regeneration in the above-mentioned wavelength region, the particle diameter of ferrite is favorably less than about 0.3 .mu.m. Since the particles of about 0.01 .mu.m in diameter do not show the desired ferromagnetism, as an preferable diameter of particle, the range of 0.01 to 0.3 .mu.m is demanded." Japanese Patent Application Laying Open No. 57-212623 (1982) discloses that the average particle diameter of magnetic barium ferrite particle is favorably below 0.3 .mu.m, and particularly, those fine particles of average particle diameter of 0.03 to 0.3 .mu.m are favorably preferable, because the particles of the particle diameter of below 0.03 .mu.m do not exhibit the sufficient ferromagnetic property necessary for magnetic recording, and on the other hand, the particles of the average particle diameter of over 0.3 .mu.m cannot advantageously carry out the magnetic recording as the high density recording.
Further, it is necessary that the value of magnetization of the magnetic particles is as large as possible, and its necessity is clearly seen in the description in Japanese Patent Application Laying Open No. 56-149328 (1981) that it is required that the saturation magnetization of magnetoplumbite-ferrite used as a material for the medium of magnetic recording is as large as possible.
On the other hand, as a method for producing barium ferrite particles, a method of treating an aqueous alkaline suspension containing Ba-ions and Fe(III) in an autoclave as a reactor (the method is hereinafter referred to as "the autoclaving method") has been hitherto known, and by selecting the reaction conditions in the autoclaving method, barium ferrite particles precipitate. The thus precipitated particles are hexagonal plate-like particles, and the distribution of the particle size and the average size of the particles differ according to the reaction conditions resulting in the difference of the magnetic properties of the particles. In the technical field of producing plate-like barrium ferrite particles for use in magnetic recording, not only the method of forming the product from an aqueous solution but also the method for forming the product from a fluid has been hitherto tried. On the other hand, a technique by which non-ferromagnetic barium ferrite particles are formed from an aqueous solution according to the autoclaving method and the thus obtained non-ferromagnetic barium ferrite particles are sintered at a high temperature to obtain ferromagnetic barium ferrite particles has been tried.
Further, as is seen in Japanese Patent Application Laying Open No. 56-149328 (1981), in order to reduce the coercive force of the barium ferrite particles, it is proposed that a part of Fe(III) in the ferrite is substituted by Co(II) and Ti(IV). In this case, as is clearly seen in Example of Japanese Patent Application Laying Open No. 56-149328 (1981), unless the large amount of Co(II) and Ti(IV) is added so that in the formula BaCo.sub.x Ti.sub.x Fe.sub.12-2x O.sub.19, becomes larger than 0.8, the coercive force cannot be reduced to the extent which is appropriate as the magnetic material for magnetic recording. However, such a large amount of Co(II) and Ti(IV) causes the reduction of the purity of the thus formed barium ferrite particles resulting in a large reduction of the magnetization although the effect of reducing the coersive force is larger. The above-mentioned fact is clearly seen in Comparative Example 1 of Japanese Patent Application Laying Open No. 56-149328 (1981) wherein the extent of saturation magnetization is as small as 26 emu/g.
The present inventors, in order to reduce the coercive force effectively without remarkably reducing the magnetization by adding Co(II) and Ti(IV) in an amount of as small as possible, have repeated the systematic investigation on the relationship between the conditions in formation of barium ferrite particles from an aqueous solution and the average particle diameter, the particle size distribution and the magnetic properties of the thus formed barium ferrite particles.
As a result of the present inventors' study, it has been found that in the case where an aqueous strongly alkaline suspension containing Fe(III), Co(II), Ti(IV) and Ba ions is autoclaved at a temperature in the range from 250.degree. to 320.degree. C., wherein the total amount of Co(II) and Ti(IV) is 0.017 to 0.09 to one atomic amount of Fe(III), the amount of Co(II) is the same as that of Ti(IV) and the atomic ratio of Ba ions to the sum of Fe(III), Co(II) and Ti(IV) ions is 1:7 to 1:9, fine plate-like particles represented by the formula, BaCo.sub.x Ti.sub.x Fe.sub.12-2x O.sub.19 wherein 0.1.ltoreq..times..ltoreq.0.5, having the properties of 20 to 70 m.sup.2 /g of BET specific surface area and 0.05 to 0.3 .mu.m of average particle diameter, and showing more than 30 emu/g of value of magnetization in a magnetic field of 10 kOe and 300 to 1,000 Oe of coercive force are formed, and based on the finding, the present invention has been attained.